Technology: The Power & Potential of Pure Light

TECHNOLOGY

The Power & Potential of Pure Light

There was no word to describe the new device when scientists first learned how to build it. But there seemed to be no limit to its potential. The fierce pure light they were coaxing out of synthetic crystals was so powerful that the military believed its long-sought super-weapon—a death ray—might finally become a reality. Applications in medicine and in industry seemed limited only by the human imagination.

Then for a while, optimism faded. Practical uses for the new source of light, which scientists christened laser (for light amplification by stimulated emission of radiation), proved to be both scarce and elusive. Physicist Theodore Maiman, an early laser pioneer, described the new light source as “a solution seeking a problem.” He was understandably impatient, but problem after problem has since been found— in ever increasing numbers. And the versatile laser is beginning to solve those problems in a manner that more than justifies the early, expansive claims. Lasers have become a $300 million-a-year business. As they are made more efficient and mass production cuts costs, the market should grow rapidly—to a billion dollars a year by 1975, according to laser experts.

Welding Retinas. Taking advantage of the unerring straightness and narrow diameter of laser beams, engineers are already using them to keep bridges, tunnels and dams in line during construction. Laser light has also proved helpful in aligning jet-plane assembly operations and the two-mile-long Stanford linear accelerator. When the high energy of laser light is concentrated on a small area, it serves as a high-speed drill that can burn precision holes through materials as hard as diamonds in a small fraction of the time required by conventional methods. It can vaporize the rough edges of such microscopically small products as integrated circuits. A less powerful laser beam can weld wires and other delicate metallic parts without damaging nearby heat-sensitive parts.

The precision and penetrating power of laser beams have, as predicted, given them entree to the operating room, where they can cut into human and animal tissue as delicately as a finely honed scalpel. Even better, the laser knife does not draw blood. Its searing but highly localized heat cauterizes capillaries and other blood vessels as they are severed. Like ordinary light, laser beams pass through transparent substances but are absorbed by darker, opaque materials. Thus they flash harmlessly through the cornea and lens of the eyeball to weld a detached retina back into place, or puncture small holes in the retina to ease the pressure of glaucoma. They also penetrate translucent skin to vaporize skin cancers, or tattoos that out-live the patient’s enthusiasm for such decoration. Lasers are being used in data pro- cessing to heat tiny, closely spaced spots on magnetic film, thus altering the magnetization and increasing the bits of information that can be packed into a given area. Laser beams themselves can be modulated to carry considerably more intelligence than any radio waves. The Air Force Avionics Laboratory at Wright-Patterson Air Force Base has developed a single laser communications system that can carry ten television channels simultaneously.

Pinch of Chromium. In theory, all this became possible in 1917, when Al- bert Einstein pointed out that an atom or molecule stimulated by an electro-magnetic wave (light, for example) would give off a basic unit of light called the photon, which would have the same wave length as the stimulating wave. A number of subsequent experiments proved Einstein correct. But not until 1958 did Physicists Arthur Schawlow and Charles Townes describe a device that they thought would be able to stimulate molecules of gas confined in a cylinder until they gave off photons in an intense and powerful stream. Their device was a variation of Townes’s earlier Nobel Prizewinning invention, the maser—an instrument that produced invisible microwaves by a process called “microwave amplification by stimulated emission of radiation.” Because it was designed to produce visible light, ihey called their proposed new instrument an optical maser.

Just two years later, Physicist Maiman used the Townes-Schawlow theory and built the world’s first working laser, a small, hand-held instrument that shot out bursts of brilliant red light. Instead of a gas, Maiman’s laser used a synthetic ruby crystal grown in a bath of molten aluminum oxide. In pure form, the aluminum oxide crystal is colorless and transparent. But a pinch of chromium added to the bath as an impurity gives the resulting crystals their characteristic ruby-red hue and supplies the chromium atoms (one for every 5,000 aluminum atoms) that cause the laser action. Excited Atoms. Both ends of the crystal rod are highly polished and silvered to act as mirrors, one highly reflective, the other partially transparent. Wrapped around the rod in the form of a coil is a flash tube similar to the strobe lights used by photographers.

When a pulse of electricity is fired through the tube, it gives off a brief, intense flash of light. Inside the ruby rod, the chromium atoms are highly excited by the light flash; their electrons temporarily absorb excess energy. Then, as the electrons fall back toward their normal energy levels, each emits a photon. Some of the photons pass through the transparent walls of the ruby rod and are lost. But many hit the mirrors at either end of the rod and are reflected back to the opposite mirror. As they bounce back and forth along the rod, they stimulate other chromium atoms into emitting photons (see diagram, p. 49). When the chromium atoms that are in an excited state become more plentiful than those that are not, a torrent of photons bursts through the partially transparent mirror at one end of the rod in a brief but intense pulse of red light, vastly more powerful than the flash that triggered it.

American Pinpoints. Unlike ordinary “white” light from an incandescent bulb, which is a mixture of all colors, and thus of many different wave lengths traveling in divergent directions, laser light is what scientists call “coherent.” It emerges from the rod in rays that are parallel; it is all of the same wave length, and it is all in phase or in step, each ray reinforcing the others, like oarsmen in a superbly trained crew.

It is these coherent qualities that make laser light so narrow-beamed, so easy to focus and so powerful. Laser light can be focused into a spot with a diameter of only I/10,000th of a centimeter. Concentrated into so small an area, it burns billions of times brighter than the sun’s surface. Instead of rapidly diverging as it moves farther from the light source—like rays from an ordinary spotlight—highly disciplined laser light remains confined within a narrow beam over remarkably great distances. In tests made several years ago, a beam from a ruby laser was aimed at the moon, 240,000 miles away. When it reached the lunar surface little more than a second later, it lit an area only about two miles in diameter. A beam of incoherent light from a standard searchlight, hypothetically powerful enough to reach the moon, would have spread much more, covering a huge area tens of thousands of miles across. Last January, Surveyor 7, sitting on the moon, actually photographed powerful laser beams aimed at it from earth. They appeared in Surveyor’s televised pictures as tiny pinpoints of light on the darkened North American continent. Such experiments, to be sure, require good weather. For powerful as it is, laser light can be blanked out by cloud or fog.

Invisible Rays. Although Maiman’s synthetic ruby was the first substance made to “lase,” it was far from the last. Some 100 different gases, glasses, plastics and liquids have since been teased into producing laser beams— some by intense flashes of light, some by high-voltage discharges, others by the injection of a stream of electrons. Laser beams can now be produced continuously or in pulses; depending on the kind of lasing material, they can be created in a spectrum of wave lengths giving them colors that range from orange to blue to invisible infrared.

Continuous lasers are generally chosen for such service as communications, which requires steady transmission. Pulsed lasers are used to best advantage in drilling, welding and surgery because of the tremendous power they can pack into each pulse. The color of the emitted light is relatively unimportant, but the military usually prefers infra-red because it is more difficult to detect. And lasers made with argon gas are used to illuminate underwater areas because their blue-green beams penetrate farther into the murk. Three-D Images. “Lasers are still very primitive devices,” says Physicist Schawlow. “They’re still about at the crystal-set stage of radios, or airplanes around 1910. Laser technology has come a long way, but it still has a hell of a long way to go.”

On that long route, lasers are already lighting the way toward ever more spectacular scientific achievements. In the laboratory, the beams have provided the coherent light necessary to produce holograms, transparencies made by photographing a subject in the light of a single laser. A beam from the laser mixes with light reflected from the subject to make an apparently meaningless pattern of lines and whorls. When properly illuminated, that pattern becomes a three-dimensional image that seems to float in space; a viewer can actually look behind an object in the picture merely by moving his head. Although many complex problems remain to be solved, some scientists are convinced that holography will some day be the source of three-dimensional television. An experimental television camera recently developed by Connecticut’s PerkinElmer Corp. uses a weak laser beam that scans a subject so rapidly that its red light is virtually invisible to the human eye. But it still provides enough reflected radiation for a fully illuminated picture on the TV screen without any additional lighting. The Army is experimenting with laser television for secret nighttime surveillance from aircraft, and military planners are developing bomb warheads that seek out targets illuminated by invisible infra-red laser beams. Peeling Potatoes. The various laser wave lengths, about 1,000 times shorter than those of the microwaves used in conventional radar, make laser altimeters, range finders and aerial mappers remarkably accurate. In a demonstration of a laser distance-measuring device, Spectra-Physics, Inc. flew the instrument over a Philadelphia high school stadium at an altitude of 1,000 ft. A conventional radar altimeter would have indicated only the slope of the stadium; the laser picked out each row of seats, the one-foot space between each row, and even the slight depression of the running track at ground level. In no more than 20 years, Physicist Schawlow predicts, the laser will be a common tool “in the office, in the factory, and in the home, where it could be used for peeling potatoes.” Or, he says, as he casually lights a book of matches with a hand-held laser, “it might even be used as a pilot light for kitchen stoves.” To prove that his predictions are not as far-out as they seem, Schawlow has built and will soon market a laser eraser, a model of which he has already attached to his own typewriter. When he makes a mistake, Schawlow merely presses a button. Zap! The laser completely vaporizes dark (and thus light-absorbing) typed words or individual letters, leaving the paper unscarred and with no rubbing to be brushed away.

Imaginative scientists have proposed that powerful laser beams (which actually exert pressure on a surface) be used to push back into correct orbit satellites that have begun to fall toward earth. Others have gone beyond the early idea of a death ray and suggested that laser beams may eventually be powerful enough to provide the ultimate defensive weapon against missiles. Powerful laser beams, they predict, might well make iCBMs obsolete. Focused on an incoming missile, their light would generate enough heat to melt it into uselessness.